The present invention relates to a method for extracting copper in liquid-liquid solvent extraction from aqueous solutions with a high sulphate content, by raising the viscosity of the extraction solution and by dispersing the aqueous solution into drops, achieving a dense drop aggregation. The viscosity of the extraction solution may be raised either by increasing the content of the actual extractant, the extraction reagent, in the extraction solution or by using a diluting agent with a higher viscosity than that of the diluting agent normally used. By raising the viscosity of the extraction solution the mixing durability of the extraction dispersion can be increased and resulting of that, the amount of residual drops is decreased. Other advantages are that the extraction solution flow of the extraction process decreases in relation to the flow of the aqueous solution acting as the copper source and that the size of the extraction equipment needed is reduced.
Dilute aqueous solutions form when poor copper ores are leached directly. The copper content of such solutions is usually of the order of 1-4 g/l Cu. In addition neutral salts often accumulate in the solution, mainly aluminium and magnesium sulphates. Although the copper content does not rise above 1.5 g/l, the sulphate content may rise above 40 g/l, to between 40 and 120 g/l. Some of the sulphate may originate from the ore or the possible use of seawater. In the extraction process the aqueous solution is in a cycle between the extraction and the leaching and thus gradually accumulates the salts which raise the viscosity. Neutral salts can easily cause a viscosity increase harmful to the aqueous solution, even 3 cP, which also disturbs the dispersing of the aqueous and extraction solutions and results in high amounts of residual drops. In particular when dispersion is desired where the organic solution is continuous and the water in drops, an increased viscosity in the aqueous solution can make it difficult to achieve such a dispersion. Previously the increased viscosity caused by neutral salts and the resulting disadvantages were not taken into account.
In copper extracting processes a mixed organic extraction solution and aqueous solution is generally used in the ratio of O/A (organic/aqueous) 1.0-1.2. Present-day copper extracting processes usually follow recommendations given by extractant manufacturers, according to which the organic and aqueous solution of the extraction O/A ratio at all extraction stages of the extraction process should be of the order of 1.0, and the extractant content raised to 3.3-4.2 vol. % per every gram of copper, which comes in the direction of flow of the first extraction stage of extraction. In practice this means that if the Cu content of the aqueous solution is 1.5 g/l, the extractant content is maximum 6.3 vol. % according to the recommendations. Generally, when the amount of copper in the solution increases, the amount of extractant decreases relatively. The type of extractant is a chelating copper complexing agent, usually hydroxyoxyme, which forms a strong complex with copper, and one fact affecting the progress of copper extraction is how much extractant is present in relation to the amount of copper to be extracted.
Generally, alifatic or aromatic hydrocarbons, kerosenes, with a distillation range between 190-245xc2x0 C. are used as the diluting agent for the copper extractant The viscosity of these substances is usually below 2 cP, and for aromatics even below 1.5 cP. It is also possible to us mixtures of aromatic and alifatic hydrocarbons as the diluting agent, where the aromatic content of the mixture is around 20-30 vol. %.
As previously stated, in copper extraction it is difficult to get a dispersion of an aqueous solution with a high sulphate content, minimum 40 g/l, where the organic phas is continuous and the aqueous solution in drops, although this is essential in order to improve extraction performance. According to th present invention, the viscosity of the extraction solution is now raised to the area of 3-11 cP, and this takes place either by raising the extractant content or by using a diluting agent with a high viscosity in the extraction solution. In addition to this, that the organic phase has been made continuous, the method has proved to have many other advantageous consequences. The essential features of the invention will become apparent in the attached patent claims.
A rise in the viscosity of the extraction solution clearly raises the mixing durability of the extraction dispersion. In this connection a mixing-durable dispersion means a dispersion where no drops below 0.2 mm appear when the mixing intensity is max. 0.15 kWh/m3 in a mixing volume of 50 m3. Volume-specific mixing power is dependent on the mixing volume so that the power required decreases slightly as the volume increases. Obviously the mixing itself also affects mixing durability. The mixers described in U.S. Pat. No. 5,185,081 have been settled on to use in the method according to the present invention. These mixers have a double helix, which helps to avoid locally increasing shear rate forces and the small drops generated as a result. When the viscosity of the organic phase has been raised according to the invention and the extraction dispersion made heavier and this dispersion combined with a very smooth, thoroughly uniform mixing of controlled intensity in the mixing area, the conditions are achieved where an evenly distributed mixing energy is not sufficient to attain a turbulence to form droplets. An evenly attenuated mixing creates a dispersion where the drop size is uniform and which thus possesses good separation characteristics. Since the amount of residual drops is small, the extraction result is clearly improved.
In addition to the increase in viscosity of the extraction solution, another key factor is the mixing ratio of the solutions. The denser the drop aggregation, the heavier and simultaneously more mixing-durable the dispersion. The most advantageous result is obtained when a dispersion is formed, where the extraction solution is continuous and the amount of water drops is raised.
When the viscosity of the extraction solution is raised, it has been found that an extraction solution with a higher viscosity is better able to keep a larger amount of the aqueous solution than normal inside it as drops. In the method according to our invention it is possible to lower the O/A ratio to between 0.7-1.0 without endangering the continuity of the extraction solution. In practice, this means that the extraction solution flow can be reduced in relation to the copper-containing feed solution (the aqueous solution) by the amount previously described. At the same time the extractant content of the extraction solution is increased to the extent that the mass flow of the extraction solution stays unchanged or increases a little. Thus the viscosity of the extraction solution can be raised successfully.
The factor by which the extractant content is raised compared with the normal recommendation in the method according to the present invention, varies between 1.2-5, and is preferably between 1.5-3. When very dilute copper-containing feed solutions with max. 2 g/l of copper are being treated, the factor may always rise to 5 i.e. according to our invention the extractant content would then be of the order of 7-25 vol. %, preferably 15-25 vol. %. When the feed solution copper content is between 2-4 g/l, the preferred extractant content is in the range of 15-30 vol. %. Generally, however, the extractant content does not increase above a content of 30% by volume. The viscosity of the extraction solution in this case rises to between 3-7 cP, which is enough to raise it to a clearly higher level than the viscosity of the aqueous solution. Normally, the aim is to achieve an O/A viscosity ratio of between 1.2-3, preferably 1.5-2. According to the invention, when extracting dilute copper solutions the extractant content in the extraction solution is presently set in the range of 7-30 vol. %, preferably 15-30 vol. %.
Regarding aqueous solutions containing over 4 g/l of copper, even an ordinary extractant content in the extraction solution gives a fairly good result. For these solutions, the use of an extractant factor of 1.2-2.0 times the recommendation improves the mixing durability of the dispersion. With the method according to the invention, however, it is possible to raise the extractant content in the extraction solution to 25-50 vol. %, when the copper content of the aqueous solution is 4-8 g/l and even up to 40-70 vol. % if the copper content of the solution is over 8 g/l. The viscosity of the extraction solution can also be raised partly or wholly with the use of a diluting agent. The distillation range and viscosity of the diluting agents generally used was mentioned earlier as being rather low. If other diluting agents are used, this can also raise the viscosity of the extractant. Alifatic hydrocarbon products can be chosen with a distillation range in the range of 220-275xc2x0 C. or 240-270xc2x0 C., and the viscosity of these substances measured at a temperature of xc2x125xc2x0 C. is 2.7 or 3.2 cP. If it is desired to use aromatic hydrocarbons, the viscosity of hydrocarbons with a distillation span of 230-290xc2x0 C. is about 3 cP. It is also possible to use mixtures of alifatic and aromatic hydrocarbons.
When treating dilute aqueous solutions containing less than 4 g/l of copper, there is a possibility in our invention of using hydrocarbon compounds that boil at a high boiling range as the diluting agent. The use of a diluting agent to increase viscosity is preferred since the diluting agent is always cheaper than the actual extractant. The proportion of diluting agent in the extraction solution can be between 30-93%. It is easier to achieve the required rise in viscosity without the density of the extraction solution increasing significantly with alifatic hydrocarbons. The use of alifatic hydrocarbons is also recommended for reasons of industrial hygiene.
It was mentioned above that when raising the viscosity of the extraction solution it is possible to decrease the external pumping of the extraction solution coming to the extraction stage from outside. If the rise in viscosity takes place with an extraction solution diluting agent, it is not possible to decrease pumping. On the other hand, when viscosity is raised with an extractant, xternal pumping of the extraction solution can be reduced significantly compared with the amount of copper-containing aqueous solution being conveyed to the extraction stage. If viscosity is raised both by increasing the extractant content and by using the aforementioned diluting agent, the amount of external pumping decreases in the same degree as the extractant content is increased.